Artificial glass sometimes crystallizes and contains spherulites that may be as large as a marble. As the glass has little similarity in chemical composition to volcanic obsidians, these spherulites when analyzed throw little light on the mineral nature of spherulites in rocks. They show, however, that in viscous semi-solid glasses near their fusion point crystallization tends to nucleate at certain centers and to spread outwards, producing spherulitic structures. Many salts and organic substances exhibit the same tendency, yielding beautiful spherulite crystallizations when melted and cooled rapidly on a microscopic slide.
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